1. Field of the invention
The invention concerns a non-equilibrium chromium-nitrogen alloy surface coating for conferring good friction properties on a substrate. The invention also concerns a method of applying such coatings.
2. Description of the prior art
The expression "friction properties" when applied to a surface or a coating refers to the properties which determine the friction behavior of the surface or coating as they evolve with time. The parameters which express this behavior are essentially the friction wear rate and the coefficient of friction.
The friction referred to here is true friction between two surfaces, and not lubricated friction where a film of lubricant is interposed between the two solid surfaces and the relative movement essentially occurs between layers of lubricant in the film.
Under severe friction conditions, with a high load per unit area, resistance to wear implies a high hardness of the rubbing surface. However, increasing the hardness of a friction coating usually increases in its fragility, with the risk of premature wear through erosion.
There is a definite correlation between wear rate and coefficient of friction because increasing the coefficient of friction, all other things being equal, leads to an increase of degraded energy and the wear rate is in direct proportion to the degraded energy.
In many applications the energy degraded by friction represents only a minor part of the energy involved; it is then more important to attempt an overall reduction in the wear rate, which depends on the hardness, on the fragility and on the coefficient of friction combined, rather than to reduce the coefficient of friction alone.
In what follows, the expression "good friction properties" means essentially good resistance to wear, including the wearing of fragile coatings by erosion. The typical test will involve measuring the rate of wear or of removal of material on a plane coating in contact with a ring of high speed steel with a load of 20 daN and a sliding speed of 0.55 m/s on the plane coating in water.
The interrelated documents FR-A-2 512 070 and EP-A-0 074 322 describe alloys with good friction properties comprising chromium and an alloying element in the form of carbon or nitrogen in metastable interstitial solution in the chromium with concentrations between a minimum representing saturation of the equilibrium solid solution of the alloying element in the chromium and a maximum representing saturation of the interstitial solution.
In the context of the present invention only the chromium-nitrogen system is considered. In this system the content of nitrogen is expressed as the number of atoms of nitrogen per one hundred atoms of chromium-nitrogen alloy.
In the thermodynamic equilibrium diagram of the chromium-nitrogen system, and moving in the direction of increasing nitrogen content, a solid solution region with 0 to 0.15 atoms of nitrogen per 100 atoms of alloy is followed by a Cr.sub.2 N nitride region with 33.3 atoms of nitrogen per 100 atoms of alloy, followed by a region of CrN nitride with 50 atoms of nitrogen per 100 atoms of alloy. The Cr.sub.2 N nitride has a hexagonal crystal structure and the CrN nitride has an NaCl type cubic structure. This CrN nitride has all the characteristics of a ceramic. Two-phase chromium-nitrogen alloys are routinely produced with a cubic chromium phase and a hexagonal Cr.sub.2 N nitride phase.
The prior art documents cited above, and the document EP-A-0 074 322 in particular, teach saturation of metastable interstitial solutions of nitride in the chromium for 3.5% by weight of nitrogen, which represents substantially 12 atoms percent of nitrogen. These documents teach that the interstitial solutions have a hardness very much greater than that of the chromium (a Vickers hardness of up to 2,200, for example, as compared with 600 for the chromium), and that for concentrations of alloying elements greater than the saturation of the interstitial solution the composition of the alloys was as predicted from the equilibrium diagrams.
These interstitial solutions are obtained by fast cathode sputtering using a chromium cathode on a substrate in a reduced pressure atmosphere consisting of a rare gas containing the alloying element in the combined dissociable state or in the free state.
Metastable interstitial solutions cause distortion of the cubic crystal lattice of the chromium, which could account for the increased hardness.
The applicant has discovered that, going against what might be expected from the prior art, it is possible in the chromium-nitrogen system to obtain metastable alloys comprising more nitrogen than the interstitial solutions mentioned above and which have better friction properties than interstitial solutions combined with a reduced wear rate, freedom from fragility and a relatively low coefficient of friction.